Two measurements commonly performed on the whole blood are the hematocrit and hemoglobin. The hematocrit is the percentage volume that packed red blood cells occupy in a centrifuged sample of whole blood, and the hemoglobin content is the weight of the hemoglobin per unit volume of whole blood. The numeric ratio of hemoglobin to hematocrit is referred to as the mean corpuscular hemoglobin concentration (MCHC), and in normal individuals it is close to 33.9%. When an individual is suffering from certain diseases, however, the ratio may vary from about 38% down to 26%. Thus, the determination of both the hematocrit and hemoglobin are important for the discovery and diagnosis of anemia or other blood disorders.
In a large laboratory the measurements of hematocrit and hemoglobin are usually made concurrently in an automated analyzer, but in a small clinic or in a physician's office, they must be made separately, using two different techniques. The hematocrit may be presently performed by filling a small bore glass tube with anticoagulated whole blood, sealing one end of the tube, and centrifuging the tube to pack the red blood cells. After packing, which takes about three to five minutes in a small centrifuge, the length of the packed red blood cell column and the total filled length are measured, and the hematocrit, expressed as a percentage, is calculated. It can be appreciated that little skill is required to prepare the tube or take the measurements. U.S. Pat. Nos. 4,027,660 issued June 7, 1977 to S. C. Wardlaw et al; 4,181,609 issued Jan. 1, 1980 to S. C. Wardlaw et al; 4,156,570 issued May, 1978 to S. C. Wardlaw; and 4,558,947 issued Dec. 17, 1985 to S. C. Wardlaw, and others describe a procedure which involves drawing a sample of anticoagulated whole blood into a capillary tube, placing a float in the tube with the blood sample, and centrifuging the blood sample to cause the float to settle into the red cell layer to elongate the buffy coat in the blood sample. This prior art technique can be used to measure hematocrit as well, by merely taking into account the expansion of a portion of the blood sample by the float when calculating the total length of the blood components, the observed total length being scaled down by the measuring instrument to compensate for the presence of the float.
On the other hand, the measurement of the hemoglobin concentration is considerably more complicated. To perform this test, in a small clinic or physician's office, the blood sample must be more accurately diluted to a ratio of either 1:250 or 1:500, depending on the equipment used. The dilution is made by accurately taking a tiny sample of the blood into a pipette and delivering it into a container containing an agent which dissolves the red blood cells, and cyanide, which converts to hemoglobin to a more easily measurable form. This mixture, after standing for three to ten minutes, is then placed in a photometer where the light attenuation at 560 nm (green) is compared to that of standard solutions. From these comparisons, the concentration of hemoglobin can be calculated. There are many published variations of this method, but all acceptable means to date require the accurate measurement of the light attenuation in an instrument designed for this purpose. Further, the need for accurately handling small quantities of the sample requires a higher level of skill than does the performance of the hematocrit, and is therefore also a source for analytical errors.